1. Technical Field
The disclosure relates to an oscillation circuit which provides an output clock signal to a processor and more particularly to an automatic self-calibrated oscillation method and an apparatus using the same.
2. Related Art
Accurate clock generators or timing references have generally relied upon crystal oscillators, such as quartz oscillators, which provide a mechanical, resonant vibration at a particular frequency. The difficulty with such crystal oscillators is that they cannot be fabricated as part of the same integrated circuit (“IC”) driven by their clock signal. For example, microprocessors require a separate clock IC. As a consequence, an off-chip clock generator is desired for a circuit requiring an accurate clock signal.
There are several consequences for such non-integrated solutions. The non-integrated solutions, where an additional IC is required, increase space and area requirements, whether on the PCB or within the finished product, which is also detrimental in mobile environments. Moreover, such additional components increase manufacturing and production costs, as an additional IC must be fabricated and assembled with the primary circuitry (such as a microprocessor).
Other clock generators which have been produced as integrated circuits with other circuits are generally not very accurate, particularly over fabrication process, voltage and temperature (“PVT”) variations. For example, ring, relaxation and phase shift oscillators may provide a clock signal suitable for some low-sensitivity applications, but have been incapable of providing the higher accuracy required in more sophisticated electronics, such as in applications requiring significant processing capability. In addition, these clock generators or oscillators often exhibit considerable frequency drift, jitter, have a comparatively low Q-value, and are subject to other distortions from noise and other interference.
As a consequence, it is necessary to develop an apparatus for automatic self-calibrated oscillation.